The present invention relates generally to a method and apparatus for aerial image formation. More specifically, it relates to an improved system for reference distribution aerial image formation for use in both microimagery and microscopy. More specifically, still, it relates to an improved system for microimagery and microscopy with both increased fixed size depth of field and no fundamental resolution limit.
The continued progress of semiconductor technology is dependent on making semiconductor devices faster and smarter. This is, in turn, dependent on shrinking the size of transistors squeezed onto a silicon wafer. For example, transistors with an electrical channel, or gate, measuring between 0.1 and 0.2 micron, and smaller, are now planned.
However, it has become increasingly difficult to make smaller transistors. At these smaller dimensions, the photographic process for developing a circuit image on the surface of a silicon wafer, photolithography, starts to falter. In photolithography, light is channeled through a mask, a transparent plate covered with a circuit pattern. The light then moves through an imaging system that demagnifies the circuit pattern and projects an aerial image of the circuit pattern onto a silicon wafer.
The separation of two points in an aerial image is limited to a certain minimum distance, related to the wavelength of the light used, known as the resolution limit. Moreover, before reaching the resolution limit, efforts to increase resolution, by increasing the effective aperture of the imaging system decrease the depth of field of the aerial image.
In addition the separation of two points in an object that can be distinguished when observed by means of optical microscopy is also limited in the same way to a certain minimum distance also known as the resolution limit. Efforts to increase resolution in microscopy also lead the same way to a decrease in the depth of field of the aerial image.
Traditionally, aerial images of a subject are formed by using a distribution of light that propagates from the subject alone. In the present invention, a distribution of lightxe2x80x94a reference distributionxe2x80x94which is usually (but not necessarily) separated from the subject is introduced. An aerial image of both the subject distribution and the reference distribution is formed. The present invention is a system for increasing both the field size and the depth of field of aerial images and removing resolution limits.
The present invention is a method and apparatus of reference distribution aerial image formation. It produces increased resolution, depth of field and field size in aerial image formation of a subject, which is used in microimagery, microscopy and other applications.
The present invention splits a coherent laser beam into an illumination beam and a reference beam. A portion of the illumination beam is passed through, or reflected off of, a subject in an object plane to create a subject distribution. A portion of the reference beam is passed through a pinhole, or reflected off a point reflector, in the object plane to produce a reference distribution. An imaging system is used to form an aerial image of the subject distribution and the reference distribution on an image plane. The resulting aerial image of the subject distribution exhibits resolution, depth of field and field size than a traditional aerial image formed of the subject distribution with the same imaging system.